Analysis of the factors influencing the acetylation rate of wood

Factors influencing the rate of acetylation were examined based on the swelling of wood in the reaction solution and the dimensions of the wood sample. The activation energy of acetylation was also estimated. In a swelling test, it was found that wood swells thoroughly in acetic anhydride even without pyridine above 60°C. Therefore, pyridine may facilitate the acetylation process as a catalyst and not as a swelling agent. The weight gain, x (%), attained at reaction time t (h), for various compositions of acetylation solution or dimensions of wood sample were analyzed by applying an original rate equation [x = a × (1 – e^–kt )^1/n ], where a is the ultimate weight gain (%), k is the rate constant (h^–1), and n is a measure of the hindrance against the diffusion of reagent. The optimum volume fraction of pyridine in the pyridine-catalyzed acetylation was about 0.2. Accompanied by a rise in pyridine content, the reaction showed increased diffusion-controlled behavior. The rate constant, which is not affected by the dimensions of the wood sample, was estimated from which an activation energy of about 130kJ/mol was calculated.     

Potassium acetate-catalyzed acetylation of wood at low temperatures II: vapor phase acetylation at room temperature

Ezomatsu wood blocks were impregnated with potassium acetate (KAc) and then exposed to acetic anhydride vapor at 25°C and 120°C. The KAc-impregnated wood was rapidly acetylated at 120°C, and only 6 min was needed to achieve 20% weight percent gain (WPG). The WPG increased with increasing catalyst loading (CL), but it turned to decrease above 20% CL probably because the diffusion of acetic anhydride vapor was hindered by excess KAc depositing in the cell lumina. Thus, careful control of CL is necessary in the vapor-phase acetylation. KAc was also effective in catalyzing the vapor-phase acetylation at 25°C: the KAc-impregnated wood attained 20% WPG within 7 days, whereas the WPG did not exceed 10% even after 1 month in the uncatalyzed system. Irrespective of treatment methods, the hygroscopicity of wood was reduced and its dimensional stability was improved with an increase of WPG. These results confirm that the use of KAc simplifies the acetylation process at room temperature with minimal loss of acetic anhydride.     

Potassium acetate-catalyzed acetylation of wood at low temperatures I: simplified method using a mixed reagent

Ezomatsu wood blocks were acetylated in a mixture of acetic anhydride and acetic acid containing excess potassium acetate (KAc). The mixture method enabled rapid acetylation at 120°C: a 20% weight gain (weight percent gain; WPG) was achieved within 30 min while the WPG did not exceed 18% after 120 min of conventional uncatalyzed acetylation. At 40°C, however, a satisfactory WPG was not achieved with the mixture method because both the wood swelling and KAc concentration in the reagent solution were limited at that temperature. In addition, the antiswelling efficiency attained by the mixture method was irregularly low, probably because of nonuniform reaction involving shrinkage of the cell lumina. These results suggest that the mixture method is not advantageous for low-temperature acetylation, whereas it enables simple and rapid acetylation at high temperature.     

Contribution of lignin to the reactivity of wood in chemical modifications I: influence of delignification on acetylation

In order to examine the contribution of wood components to the acetylation of wood, we acetylated wood meal that had been partially delignified. The results were analyzed in terms of the reaction kinetics. The first-order rate equation was successfully adjusted to the weight gain data. The rate constant for acetylation initially increased with progress of lignin elimination and then turned to decrease; the apparent activation energy showed the reverse tendency and ranged from about 90 to 130 kJ/mol. These results suggest that lignin elimination brings not only separation of lignin but also drastic change of the chemical and/or physical structure in the residual lignin, and this affects the reactivity of wood meal as a whole. The ultimate weight gain estimated by the regression of the rate equation showed a minimum when lignin was moderately eliminated, which was explained in terms of enhanced reactivity of lignin and lower accessibility for holocellulose than predicted. The equilibrium moisture content had a maximum when lignin was moderately eliminated. This tendency is the opposite of that observed for the ultimate weight gain, and suggests that the sites for acetylation do not always correspond to those for moisture adsorption. Part of this report was presented at the 54th Annual Meeting of the Japan Wood Research Society, Sapporo, August 2004     

Swelling of acetylated wood II: effects of delignification on solvent adsorption of acetylated wood

To clarify the role of lignin in the affinities of acetylated wood for organic solvents, the effects of delignification on the solvent adsorption of acetylated wood were investigated. Acetylated wood meals rapidly adsorbed organic solvents that were hardly adsorbed by unmodified wood. For nonpolar and low-polarity organic solvents, a clear positive correlation was observed between the amount of adsorption and the lignin content. This indicated that acetylated lignin was responsible for the excellent affinities of acetylated wood for hydrophobic organic solvents. On the other hand, for lower alcohols and water, the amount of adsorption reduced with an increase in the lignin content. It was suggested that the adsorption of such polar solvents was dominated by insufficiently acetylated hydrophilic polysaccharides.     

杨木粉末温压成形本构方程的构建

为探寻木质粉末在温压成形过程中的"应力-应变"关系,基于杨木废单板制备的杨木粉末的温压成形实验数据,以川北公式为模型,借助模拟退火优化算法反演模型参数,构建杨木粉末温压成形本构方程。结果表明:构建的本构方程精度较高,本构模型的模拟结果与试验结果高度重合,可作为木质粉末温压成形机理分析及失效成因理论判据。     

Dimensional stability of wood acetylated with acetic anhydride solution of glucose pentaacetate

Five wood species were acetylated with acetic anhydride (AA) solution of glucose pentaacetate (GPA) at 120°C for 8h, and the effect of GPA on the dimensional stability of the acetylated wood was investigated. Some GPA was introduced into the wood cell wall during acetylation. The GPA remaining in the cell lumen penetrated the cell wall effectively after heating to more than 140°C for 10min. The bulking effects of GPA resulted in a 10%–30% increase in the anti-swelling efficiency of the acetylated wood with 20% GPA/AA solution in place of AA. Hydrophobic GPA did not deliquesce under highly humid conditions and it remained in the cell wall after boiling in water.Part of this paper was presented at the 51st Annual Meeting of the Japan Wood Research Society, Tokyo, April 1988     

Swelling of acetylated wood I. Swelling in organic liquids

To investigate the affinity of acetylated wood for organic liquids, acetylated yezo spruce wood specimens were soaked in various liquids, and their swellings were compared to those of untreated specimens. The acetylated wood was rapidly and remarkably swollen in liquids having low hydrogen bonding power such as benzene and toluene in which the untreated wood was swollen only slightly or very slowly. On the other hand, the swollen volume of wood in water, ethylene glycol, and alcohols remained unchanged or slightly decreased after the acetylation. The effect of acetylation was greater in liquids having smaller solubility parameters. The easier penetration of aprotic organic liquids into the acetylated wood was considered to be due to the reduction of polarity and the scission of hydrogen bonds in the amorphous wood constituents where the hydrophilic hydroxyl groups were substituted by hydrophobic acetyl groups.     

Chemical modification of wood by anhydrides without solvents or catalysts

Acetylated, propionylated, butyrylated, isobutyrylated, and hexanoylated woods were prepared at several temperatures. The reaction rate, dimensional stability, and changes in the ratios of specific gravity and dimensions in the tangential and radial directions were estimated. The reaction rate of propionylation was slow at temperatures under 90°C but increased with the temperature. The butyrylated, isobutyrylated, and hexanoylated woods showed little or no weight percent gain (WPG) and little or no antiswelling efficiency (ASE) below 110°C even for 24h, but they achieved significant WPG and ASE values at 140°C with a longer reaction time. The acetylated, propionylated, and butyrylated woods showed almost the same values for dimensional stabilization efficiency based on WPG (DSE). The specific gravity and dimensions ratios for acetylated, propionylated, and butyrylated woods compared to those of untreated wood increased with an increase in WPG.     

Mechanism of partial fixation of compressed wood based on a matrix non-softening method

Abstract

Three different mechanisms to explain the partial fixation of the compressive deformation of wood are postulated: non-softening, cross-linking and stress relaxation. This study attempted to fix the compressive deformation of wood by the non-softening mechanism of the cell-wall matrix using acetylation of the cell wall making it more hydrophobic. In this method, partial recovery of compressive deformation by wetting decreased at room temperature as the acetyl content increased. However, almost complete recovery occurred by boiling the compressed wood in water or soaking in acetone. This is due to the ability of boiling water or acetone to soften the cell-wall matrix of acetylated wood enough to enable recovery from compression. It is, therefore, possible to partially fix the compressive deformation of wood, preventing the resoftening of the cell-wall matrix in water.     

Effect of acetylation on the chemical composition of hornbeam (Carpinus betulus L.) in relation with the physical and mechanical properties

Common hornbeam (Carpinus betulus L.) is a highly underused wood species despite its great hardness, strength, wear-resistance and toughness. It is mainly used as firewood in Hungary because of its wood defects, irregular shape and low-dimensional stability. These wood defects and small breast height diameter result in a low yield. It is non-durable outdoors as it tends to turn grey, crack and be attacked by wood-decaying organisms. Indoors it lasts for hundreds of years. One technology that could improve the stability and durability properties is acetylation. Hornbeam was acetylated with the Accoya® method under industrial conditions. The aim of this research was the assessment of acetylation affecting the chemical properties of hornbeam wood and how these are related to the change in physical and mechanical properties. Main wood constituents (cellulose, hemicellulose, Klason lignin, extractives and ash content) were determined and compared. Chemical parameters related to the degradation of structural polymers were also evaluated (total phenolic and soluble carbohydrate contents, pH and buffering capacity, furfural, levulinic acid, formic acid, acetic acid). Structural changes in acetylated wood and in the Klason lignin fraction were also assessed using FTIR spectroscopy.     

木材液相乙酰化处理及处理材尺寸稳定性和耐腐性的研究

本文论述了马尾松、白桦和青杨三种木材的液相乙酰化条件及各种乙酰化木材的尺才稳定性和耐菌腐能力。以乙酸酐和二甲苯为处理液,反应的最佳条件是:温度120℃,时间7到11小时,乙酸酑与二甲苯配比1:1。当处理试样的增重到15%时,抗收缩率可达60%以上。增重为11.75—17.98%的不同处理试块,其耐菌腐能力可提高3—34倍。     

FTIR-PAS study of light-induced changes in the surface of acetylated or polyethylene glycol-impregnated wood

The objective of this study was to characterize the surface changes in acetylated and polyethylene glycol (PEG)-impregnated wood caused during light irradiation by Fourier transformed infrared photoacoustic spectroscopy analysis to determine their effects on the reduction of light deterioration. Light irradiation made the color of the chemically modified wood lighter or more vivid, whereas it deepened the color of the untreated wood. The color difference during light irradiation was less in the chemically modified wood than the untreated wood. The color difference of PEG-impregnated wood increased with increasing irradiation time. The light irradiation generated much carbonyl and significantly degraded lignin in the untreated wood. The generation of carbonyl and lignin degradation diminished in the acetylated wood in comparison with the untreated wood, indicating that acetylation restrained the photochemical degradation of wood. Deacetylation did not occur during light irradiation of the acetylated wood. The PEG impregnation decreased the generation of carbonyl and degradation of lignin during light irradiation. However, the irradiation occurred a little photochemical degradation of PEG, generating the carbonyl. Therefore, longer light irradiation should increase the degradation of PEG, thus reducing the effect of treatment. The correlation between the color difference and lignin degradation was high, indicating that the color changes during light irradiation significantly depended on lignin degradation. The chemical modification reduced the degradation of lignin and consequently decreased the color difference. Some of the compounds containing the carbonyl generated during light irradiation were water-soluble.This paper was presented at the 50th Annual Meeting of the Japan Wood Research Society, Kyoto, April 2000     

Dependence of reaction kinetics and physical and mechanical properties on the reaction systems of acetylation I: reaction kinetic aspects

The dependence of the reaction parameters of acetylation on the reaction mixture was compared among uncatalyzed, acetic anhydride-xylene mixed, and acetic anhydride-pyridine mixed solutions. Wood meal and blocks were used to examine the effect of sample size. A first-order rate equation was applied to the data, and a rate constant and leveling off value of weight gain (WG) were estimated. The rate-determining step was examined from the viewpoint of activation energy. The results were as follows: (1) Regarding the magnitude of the rate constant, the order was pyridine system > uncatalyzed system > xylene system. (2) The ultimate value of WG was lower in the uncatalyzed and xylene systems than the pyridine system, probably because of the swelling ability of pyridine. (3) The activation energies of acetylation estimated for wood meal were 120, 135, and 110kJ/mol for the uncatalyzed, xylene, and pyridine systems, respectively. (4) The characteristics of the diffusion-controlled reaction became marked when the acetylation was carried out in the pyridine system, at elevated temperature, and for wood blocks. Under these conditions, the supply of reagent to the reaction site might not be sufficient to fuel the reaction.Part of this report was represented at the 52nd Annual Meeting of the Japan Wood Research Society, Gifu, April 2002     

Dependence of reaction kinetics and physical and mechanical properties on the reaction systems of acetylation II: physical and mechanical properties

Acetylation of wood was carried out in acetic anhydride only, acetic anhydride/xylene 1:1 (v/v), and acetic anhydride/pyridine 4:1 (v/v) solutions. The antishrink efficiency (ASE), hygroscopic properties, vibrational properties, and bending strength were compared among the three reaction solutions. The ASE was a simple function of weight gain (WG); the equilibrium moisture content at a given WG differed among the reaction solutions. Based on this fact and the results of repeated water soaking and oven-drying tests, it was found that the bulking effect was a major factor, and that decreased hygroscopicity contributes only slightly to the dimensional stabilization by acetylation. The difference in equilibrium moisture content among reaction solutions appears more significant in block samples than wood meal, probably due to the fiber-to-fiber bonds in the former. The tendencies for change in the specific Youngs modulus and the loss tangent differed among reaction solutions, whereas in the static bending test the difference was not marked.Part of this report was represented at the 52nd Annual Meeting of the Japan Wood Research Society, Gifu, April 2002     

Migration of methylene bis thiocyanate in wood and its effect on a wood degrading fungus

The mobility and concentrations of methylene bis thiocyanate (MBT) at different depths of wood billets (200 mm long, 40 mm in diameter) of Pinus radiata were determined using NMR spectroscopy and ICP-ASE. Results were then correlated with the effect of wood MBT concentrations on growth of O. floccosum in a bioassay using stereomicroscopy and confocal laser scanning microscopy (CLSM). The ¹³C NMR spectra showed the presence or the absence of MBT in wood but it was unable to detect small concentrations. ICP-ASE, however produced quantitative data across the depth (40 mm) of the wood billets tested. Within 7 days of storage, MBT penetrated from surface to pith but showed significantly higher concentrations of MBT in surface wood compared to pith wood. Present study highlighted the benefit of using CLSM for fungal detection in wood. The information obtained from ICP-ASE and CLSM analysis suggested to inhibit growth of O. floccosum, the MBT concentration needs to be >55 μg/g of dry wood. Due to its high detection capacity, ICP-ASE is identified as an excellent tool for MBT detection and quantification in wood.     

Field test of resistance of modified wood to marine borers

Abstract

Five mineral fillers were tested for wood–plastic composites (WPCs): calcium carbonate, two different types of wollastonite, soapstone and talc. The impact of the fillers on the mechanical properties of the composites was studied. The experiments included bending tests, tensile tests, Brinell hardness and scanning electron microscopy experiments. The amount of wood, mineral and plastic (polypropylene) was kept steady. Only the mineral type was changed during the tests. A control sample without any mineral added was also manufactured. The mineral addition improved the tensile strength of the WPCs. The hardness of the composite was also improved when the minerals were added, and along with the increasing mineral hardness, the hardness of the composite increased. The wollastonite acicular shape was crushed during the manufacturing process, so the phase of the process in which the minerals are added requires careful consideration.     

Methodology to assess both the efficacy and ecotoxicology of preservative-treated and modified wood

? Wood used in outdoor conditions out of ground contact is susceptible to weathering, inducing both fungal decay and leaching of components to the environment.

? This paper presents a methodology to determine these two parameters for untreated, preservative-treated and modified wood. Therefore, the wood was first leached and subsequently exposed to fungal decay of the most prominent wood-rotting fungi. The crustacean Daphnia magna was exposed to the leachates to provide information on their impact on the environment.

? Combining both parameters reveals that preservative-treated wood and modified wood are capable of protecting the wood adequately for application under use class 3 conditions without posing a threat to the environment.

? This proves the suitability of the concept of combining efficacy and ecotoxicology for the evaluation of new types of wood treatments.

     

Shape stability of modified engineering wood product subjected to moisture variation

Abstract

A modified softwood product would enable the utilization of softwood in new areas. Densification is an old modification method to improve wood properties such as hardness and resistance to abrasion. A major problem with densified wood is, however, its ability to retain its original dimensions under the influence of moisture. Therefore, this study investigated the influence of surface to bottom layer thickness ratio on the shape stability of a modified and three-layered cross-laminated engineering wood product (EWP) subjected to moisture variations. The study describes a simple solid wood densification technique based on compressing a clear solid piece of softwood with vertical annual rings in the radial direction by restraining the tangential expansion. Three-layered cross-laminated EWP was manufactured with the densified wood as a surface layer. The recovery of the densified wood in the surface layer was then reduced to movements in the same level as the other two layers of unmodified wood. The EWP was subjected to climatic variations in order to investigate its shape stability. The results disclosed that an appreciable degree of shape stability was obtained by an increase in the surface to bottom layer thickness ratio of the EWP.     

Application of near-infrared spectroscopy to wood discrimination

 This study deals with a new nondestructive discriminant analysis by which wood can be classified on the basis of a combination of near-infrared (NIR) spectroscopy and Mahalanobis' generalized distance. Its accuracy and reasonability were examined for wood samples with various moisture contents ranging from oven-dried to a fully saturated free water state. In a discriminant analysis employing second derivative spectra, each wood group was well distinguished. Mahalanobis' generalized distances between softwoods are relatively independent of analytical pattern, whereas the distances between hardwoods are large for easy classification. There may be two reasons for selecting a wavelength: (1) when the chemical component of wood substance relates to the discriminant analysis; and (2) when the difference in moisture content with wood species relates to them. When we correctly construct the database of NIR spectra, confirming the purpose of the analysis, suitable wood discrimination should be possible. Received: January 23, 2002 / Accepted: March 15, 2002 Acknowledgment The authors sincerely thank the Tanabe Southeast Asia Nations Friendship Foundation for financial support. Part of this report was presented at the 51nd Annual Meeting of the Japan Wood Research Society, 2001 Correspondence to:S. Tsuchikawa